The Quantum Calorimeters Group in the Quantum Sensors Division develops and applies sensors that detect the energy of single photons or particles. For example, Transition-Edge Sensors (TESs) are able to measure the energy of single x-ray and gamma-ray photons with a precision better than one part per thousand. When assembled into arrays, TESs combine good spectral resolving power and good collecting efficiency in a way that is not possible with other detector technologies. Application areas include materials analysis, astrophysics, microelectronics defect analysis and supply chain verification, and nuclear security.